A detailed sensitivity analysis of the Iterative Maximum Likelihood Method (IMLM) algorithm and its application to the SCIAMACHY 2.3 µm spectra is presented. The sensitivity analysis includes a detailed assessment of the impact of aerosols in the 2.3 µm range. Results show that near strong aerosol sources mineral dust and biomass aerosols can have an effect of ∼7-10% on the CH 4 total columns retrieved from this wavelength range if aerosol scattering is neglected in the retrieval algorithm.

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... lar but somewhat larger effects are found for CO, but due to the larger variability of CO these errors are less important. Away from strong sources much smaller effects of a few percent are found. Using CH 4 as a proxy for CO and/or including aerosol information in the retrieval algorithm significantly reduces these errors for both CO and CH 4 . Spectroscopic uncertainties are mostly negligible except for uncertainties in the CH 4 intrinsic line intensities, which can be important. Application of the IMLM algorithm to the SCIAMACHY 2.3 µm spectra shows that the quality of the retrieved CO and CH 4 total columns is good, except for a bias for large instrument-noise errors which is partly due to remaining calibration issues. Polarization sensitivity of the SCIAMACHY instrument has a negligible effect on the retrieved CO and CH 4 total columns. The H 2 O total columns, which have to be retrieved simultaneously with CO and CH 4 due to overlapping absorption lines, agree well with H 2 O total columns from ECMWF data. This ensures that the fit to the H 2 O absorptions is of sufficient quality not to hamper the retrieved CO and CH 4 total columns from SCIAMACHY spectra. Correspondence to: A. M. S. Gloudemans (a.gloudemans@sron.nl)